Color photography



Nov. 9, 1937.

S. D. THREADGOLD COLOR PHOTOGRAPHY Filed Oct. 19, 1 935 `2 Sheets-Sheet l nummmwn IIIIHIIII Imm@ fm1@ m Nov. 9, 1937. s. D. THREADGOLD COLOR PHOTOGRAPHY 2 Sheets-Sheet 2 Filed Oct. 19, 1935 Ilma/ifm' ,JMSWJMM 9 *MMM azzfa.

Patented Nov.` 9, 1937 einer, Chroma bymesne Limited,

allgnmen Aldwyh. London,

t0 DIIU' England man om is, issn, ssi-isi No. isses'. v In Great Britain November 3, 1984 schim. (cuss-1s) l This invention 'consists of improvements in or relating to color photography and relates particularly to the reproduction ci' multi-color photographic records (referred to asthe master .5 record) of the type associated with a regular pattern multi-color screen (referred to as the master screen) by contact printing on to a photographic light sensitive inateriai- (referred to as the copy) also associatedV with a multi-color l0 screen (referred to as the copy screen) which may or may not be of regular form and may or may not be similar to that associatedwith the master record.

It is well known that difficulties arise with me 1 15 reproduction vof color photographs .in this manner due to the production oi moire patterning on the copy which gives very undesirable eilects on the copy picture. This moire patterning would seem 'to be dueto the tact that as it-isimpossible 1in practice to ensure exact registration between the elements of the same color in the master andv copy screens, the elements are 4 sometimes in phase in which case light is transp mitted by the screens formingl a light area, which 25 will ailect the emulsion, and are sometimes out oi phase in which case no light is transmitted forming a dark area which will leave the sensitive copy emulsion unaffected. It is an object o!l the present invention to reduce or eliminate Athis -so effect, which is describedI in more without undue loss of definition'.`

n im been iound'tnat ii the ugnt'pwed by the colored elements o; the master-screen is dia' tributed over the copy screen so sarto give sub- 35 stantially even illumination oi thecopy screen,

then the elements-oi the two screens 'cannot'.'in eiect, become .out of phase -as described above and light anddark areas cannot. be formed, except'as dictated by the picture itself. the eect 40 of whih is neglectedthroughout this speciiication for simplicity in explanation.

" Ifthe two color'screens are arranged in actual.

light is directed on to the master screen and- 5,6 at which thevcross sectional area ofthe light this even illumination but inbeam is constrained to dimensions so co-related with the distance between the said station and the master screen, with the size of the screen elements and with v'the distance between the master and copy screens as to give substantially uniform'illumination'of the copy screen.v

'I'he cross sectional area of the light beam may be constrained by selecting a light source of suitable area or it` may 4be constrained by inserting amagkofsuitableareainthepathofabeamot light. i

-The invention and the underlying theory will now be explained, by wayof example, with reierence to the accompanying diagrammatic drawings, in whichz- Figure 1 illustrates the eii'ect produced when two screens composed -of lines of two diilere'nt' colors are superpoeed slightly out of Figure'Z represents a known form of threecolor screen, Figure 3 shows the arrangement ot the several elements during the printing,

Figures 4a, 5a, 8a and 'la show the distribution ot the light intensities on the copy screen produced by light from a light source ot'nnite dimensions at differentdlstances from the screen and passing throughs master screen of equal transparent and opaque portions,

Figures 4b, 5b, 6b and 7b show the actual liht intensities produced on -the copy screen with the v ous' arrangements shown in Figures 4a to la tively, and

Figures 8er-and 8b show the optimum conditions otqdistribution and actual intensity respectively purposes of explanation, `it will be seen that ii',

asl shown, the lines-ot the two screens are 'not' exactly parallel, then at'the places where a red llne'oi the upper screenis over a red line of the screen underneath. .as at 3, tissscombined screens will transmit red light.. other hand. where a red. line of the upper screen isv over 'a blue line of the lower screen as at l, the combined screens willbe opaque to lights of all colors. If-r the lines. of the two screens are very nearly parallel, the transparentand opaque areas I 'and I will bands of considerable length, and it is these iight and dark bands which give the In Figure 3 the light source 5 is constituted by a mask behind which may be placed a diusing screen, not shown, and the light'passes through the master screen 6 on to the copy screen .I and then on to the copy emulsion.

Turning now to Figures 4a to 7a vand ,4b to 7b, and still considering the case of the simple twoline screen of red and blue bands of equal width, the master screen may be represented by alternate opaque and clear portions so far as either red or blue light is. concerned and is illustrated in this way at t (Figure 4a) where the clear portions are taken as the. red bands transmitting red light and the opaque portions as the blue bands transmitting substantially no red light.

If now the screen t is illuminated with parallel light perpendicular to its plane and considering only the light transmitted by the red bands, this light will produce, on the copy screen t, red bands of the same width as the master bands and the conditions described in relation to Figure l.A` will be produced, giving moire effects.

If, however, a light source offinite dimensions giving diused light is employed at a suitable distance, the red bands formed on the copy screen will be of greater width than the master bands, and the eiect will be as shown in Figure da where lthe curves it represent the intensity of the light on the copy screen, and it will be seen that this intensity is a maximum at the centre and diminishes in intensity towards the edges. The width of the bands formed on the copy screen and the intensity across the bands will vary with the angle subtended by the light source at the master screenl elements, and Figures da and 'la represent the effect for diderent angles which increase from `Figure da to Figure 7a.

In the cases illustrated in Figures- 5c to '7a the bands formed on the copy screen by the light transmitted by adjacent `master bands overlap as y represents thel copy screen'and it a curve giving the intensity of light atjany point across the copy screen.

In the arrangement shown in Figures iial and 6b the resultant intensity, as shown by the curve |12, is constant over the copy screen, and this is the theoretically correct condition for avoiding l moire.

Figures 8a and 8b show the theoretically correct conditions when the transparent elements are half the width of the opaque elements.v

Although this description has been directed to the light transmitted by the red bands, the same conditions will of course apply for ,the light transmitted by the blue elements of the master screen.

-It is found in practice thatthe best conditions are produced when the angle subtended by the light source at the master' screen is slightly less. than the theoretically ideal arrangement shown of longitudinal rec lines' i4 of que una width 4 obtained. v

' by moving it to and fro in a direction perpendicular to the plane of the film until the best effect is The mask is illuminated uniformly over its area, for example by means of an lordinary diffusing screenuniformly illuminated by any type of light source-filtered with narrow cut lters or .otherwise or, for example, produced by the method described in United Statesv Patent No. 2,024,522, issued December 17, 1935.

If, as indicated in Figure 6a, the separation between the two screens is d ,and u is the distance on either side of the element under consideration over which the light is to be spread, then the half angle tn g-g z d `from which i Thus for any given conditions the angle to be subtended at the screen `.by the light source may 'easily be calculated and therefore the dimensions of the light source for a given distance or the vdistance for given dimensions may easily be found. When the screen is unsymmetrical,` as shown in Figure 2, the dimensions of the light source may be calculated to give the desired result in each direction. It will be appreciated thatl in the case of a screen suchas that shown in Figure 2, having a series of elements of dierent size, there will be a slight unevenness in the illumination from the larger units ifthe illumination is made even for the smaller units. On. a screen having a red line, one unit wide and a green and blue line 2 units wide, the size of the aperture necessary to spread the red line evenly over the blue area and the green area, to meet the spread of light from the next adjacent red 6=2 tan line, as shown in Figures 6a and 6b, will be larger of the green and blue units this will produce a slight unevenness ofiliumination'in a direction perpendicular to the red lines, similar to that shown in Figures "7a and 7b. In actual practice,v

however, this unevenness is very slight and is unnoticeable in the nal print.

I claim:-

light source passing through the master; the effective diffusion area of the light source, its distance from the master, and the spacing of the master and copy being such that an image oi' each screen element of one color on the master is enlarged on the photographic copy to overlap adjacent color elements and overlap adjacent images of the same color to the extent that the sum of the light intensities at the overlapping portions of the images substantially equals the intenslty at the central portions of the images, thus producing substantially even illumination over the whole copy surface.

2. Apparatus for the contact printing of multicolor photographic master originals of regular pattern multi-color screen type on to photographic copy material of multi-color screen'type which comprises a source of diiIused light spaced from the master, a mask determining the etlective diffusion area of the light source, and photographic copy material spaced from the master to receive light from the light source passing through the master; the effective diusion area of the light source as determined by the mask, its distance from the "master, and the spacing of the master and copy being such that an image.

of each screen element oi' one color on the master is enlarged on the photographic copy to overlap adjacent color elements and overlap adjacent images of the same color to the extent that the surnof the light intensities at the overlapping portions of the images substantially equals the intensity at the central portions of the images. thus producing substantially even illumination over the whole copy surface,

3. Apparatus for the contact printing of multi-color photographic master originals o! regular pattern multi-color screen type on to photographic copy material of multi-color screen type which comprises a source of diffused light spaced from the master, and multi-color screen photographic copy material spaced from the master to receive light from the light source passing through the master; 'the area of the light source, the distance of the light source from the master, and spacing of the master and copy being such that an image of each screen element of one color on the master is enlarged on the photographic -copy to overlap adjacent color elements and overlap adjacent images of the same color to the extent that the sum of the light intensities at the overlapping portions of the images substantially equals the intensity at the central portions of the images, thus producing substantially even illumination over the whole copy surface.

4. Apparatus for the contact printing of multicolor photographic master originals of regular pattern multi-color screen type on to vphotographic copy material of multi-color screen type which comprises a source of diiused light spaced from the master, and photographic copy material spaced from the master to receive light from the light source passing through the master; the dimensions of the effective diiusion-area of the light source, its spacing from the master and the spacing of the master and copy being substantially in accordance vwith the formula 1u 0--2 tal'l *a I where 0 is the angle subtended b y the light source at the point on the master screen perpendicularly opposite to the center oi' the light source, u is taken to be the distance to either side of the element under consideration over vvhich the light passed by that element must .spread in orvder that the marginal portions of adjacent images of each color may overlap to such extent that the sum oi' the light intensities at the overlapping portions substantially equals the intensity at the central portions o! the images thus producing substantially even illumination over the whole copy surface and d is the separation between the master and copy screens.

STANLEY DENNIS THREADGOLD. 

